Transepithelial Ca2+ and Mg2+ transport in the cortical thick ascending limb of Henle's loop of the mouse is a voltage-dependent process

Magnesium reabsorption in the kidney

Mg²⁺ is essential for many cellular and physiological processes, including muscle contraction, neuronal activity, and metabolism. Consequently, the blood Mg²⁺ concentration is tightly regulated by balanced intestinal Mg²⁺ absorption, renal Mg²⁺ excretion, and Mg²⁺ storage in bone and soft tissues. In recent years, the development of novel transgenic animal models and identification of Mendelian disorders has advanced our current insight in the molecular mechanisms of Mg²⁺ reabsorption in the kidney. In the proximal tubule, Mg²⁺ reabsorption is dependent on paracellular permeability by claudin-2/12. In the thick ascending limb of Henle's loop, the claudin-16/19 complex provides a cation-selective pore for paracellular Mg²⁺ reabsorption. The paracellular Mg²⁺ reabsorption in this segment is regulated by the Ca²⁺-sensing receptor, parathyroid hormone, and mechanistic target of rapamycin (mTOR) signaling. In the distal convoluted tubule, the fine tuning of Mg²⁺ reabsorption takes place by transcellular Mg²⁺ reabsorption via transient receptor potential melastatin-like types 6 and 7 (TRPM6/TRPM7) divalent cation channels. Activity of TRPM6/TRPM7 is dependent on hormonal regulation, metabolic activity, and interacting proteins. Basolateral Mg²⁺ extrusion is still poorly understood but is probably dependent on the Na⁺ gradient. Cyclin M2 and SLC41A3 are the main candidates to act as Na⁺/Mg²⁺ exchangers. Consequently, disturbances of basolateral Na⁺/K⁺ transport indirectly result in impaired renal Mg²⁺ reabsorption in the distal convoluted tubule. Altogether, this review aims to provide an overview of the molecular mechanisms of Mg²⁺ reabsorption in the kidney, specifically focusing on transgenic mouse models and human hereditary diseases.

Endocrine sequelae of hematopoietic stem cell transplantation: Effects on mineral homeostasis and bone metabolism

Hematopoietic stem cell transplantation (HSCT) is an established therapeutic strategy for the treatment of malignant (leukemia and lymphoma) and non-malignant (thalassemia, anemia, and immunodeficiency) hematopoietic diseases. Thanks to the improvement in patient care and the development of more tolerable conditioning treatments, which has extended the applicability of therapy to the elderly, a growing number of patients have successfully benefited from HSCT therapy and, more importantly, HSCT transplant-related mortality has consistently reduced in recent years. However, concomitantly to long term patient survival, a growing incidence of late HSCT-related sequelae has been reported, being variably associated with negative effects on quality of life of patients and having a non-negligible impact on healthcare systems. The most predominantly observed HSCT-caused complications are chronic alterations of the endocrine system and metabolism, which endanger post-operative quality of life and increase morbidity and mortality of transplanted patients. Here, we specifically review the current knowledge on HSCT-derived side-effects on the perturbation of mineral metabolism; in particular, the homeostasis of calcium, focusing on current reports regarding osteoporosis and recurrent renal dysfunctions that have been observed in a percentage of HSC-transplanted patients. Possible secondary implications of conditioning treatments for HSCT on the physiology of the parathyroid glands and calcium homeostasis, alone or in association with HSCT-caused renal and bone defects, are critically discussed as well.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC; OMIM 248250) is a rare autosomal recessive kidney disease caused by mutations in the CLDN16 or CLDN19 genes encoding the proteins claudin-16 and claudin-19, respectively. These are involved in paracellular magnesium and calcium transport in the thick ascending limb of Henle's loop and account for most of the magnesium reabsorption in the tubules. FHHNC is characterized by hypomagnesaemia, hypercalciuria, and nephrocalcinosis, and progresses to kidney failure, requiring dialysis and kidney transplantation mainly during the second to third decades of life. Patients carrying CLDN19 mutations frequently exhibit associated congenital ocular defects leading to variable visual impairment. Despite this severe clinical course, phenotype variability even among siblings has been described in this disease, suggesting unidentified epigenetic mechanisms or other genetic or environmental modifiers. Currently, there is no specific therapy for FHHNC. Supportive treatment with high fluid intake and dietary restrictions, as well as magnesium salts, thiazides, and citrate, are commonly used in an attempt to retard the progression of kidney failure. A kidney transplant remains the only curative option for kidney failure in these patients. In this review, we summarize the current knowledge about FHHNC and discuss the remaining open questions about this disorder.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na+ -K+ -Cl- cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na+ , K+ , and Cl- ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na+ , Cl- , Ca2+ , Mg2+ , and HCO3- loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na+ -K+ -Cl- cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Medullary and cortical thick ascending limb: similarities and differences

The thick ascending limb of the loop of Henle (TAL) is the first segment of the distal nephron, extending through the whole outer medulla and cortex, two regions with different composition of the peritubular environment. The TAL plays a critical role in the control of NaCl, water, acid, and divalent cation homeostasis, as illustrated by the consequences of the various monogenic diseases that affect the TAL. It delivers tubular fluid to the distal convoluted tubule and thereby affects the function of the downstream tubular segments. The TAL is commonly considered as a whole. However, many structural and functional differences exist between its medullary and cortical parts. The present review summarizes the available data regarding the similarities and differences between the medullary and cortical parts of the TAL. Both subsegments reabsorb NaCl and have high Na+-K+-ATPase activity and negligible water permeability; however, they express distinct isoforms of the Na+-K+-2Cl−cotransporter at the apical membrane. Ammonia and bicarbonate are mostly reabsorbed in the medullary TAL, whereas Ca2+and Mg2+are mostly reabsorbed in the cortical TAL. The peptidic hormone receptors controlling transport in the TAL are not homogeneously expressed along the cortical and medullary TAL. Besides this axial heterogeneity, structural and functional differences are also apparent between species, which underscores the link between properties and role of the TAL under various environments.

Sodium Dichloroacetate Pharmacological Effect as Related to Na-K-2Cl Cotransporter Inhibition in Rats

The study objective was to investigate a possible sodium dichloroacetate (DCA) pharmacological mechanism causing an increase in diuresis in rats. The aim was to define characteristics of 24-hour urinary Na⁺, K⁺, Cl^-, Ca²⁺, and Mg²⁺ excretion in Wistar male rats and to evaluate effect of a single-dose DCA and repeated DCA dosage on diuresis. Six control and 6 DCA-treated male rats aged 5 to weeks after a single DCA dose and repeated dosage were tested. The single DCA dose treatment caused a significantly higher 24-hour diuresis when compared to control (P < .05), and it was related to increased Cl^-, Na⁺, and K⁺ urine excretion and a significant increase in Ca²⁺ and Mg²⁺ excretion (P < .05); after the repeated 4-week DCA dosage, the diuresis was not increased, but the excretion of the Na⁺, Cl^-, Ca²⁺, and Mg²⁺ ions was significantly higher. Kidney immunohistochemistry has revealed that DCA continuous treatment results in an increase in the size of Henle loop thick ascending limb epithelial cells (P < .001). The study results show a significantly reduced RNA expression of Na-K-2Cl co-transporter (NKCC1) in thymus of 4-week DCA-treated rats (P < .03). The study data have indicated a possible mechanism of such pharmacological effect to be NKCC inhibition.

Magnesium Handling in the Kidney

Magnesium is a divalent cation that fills essential roles as regulator and cofactor in a variety of biological pathways, and maintenance of magnesium balance is vital to human health. The kidney, in concert with the intestine, has an important role in maintaining magnesium homeostasis. Although micropuncture and microperfusion studies in the mammalian nephron have shone a light on magnesium handling in the various nephron segments, much of what we know about the protein mediators of magnesium handling in the kidney have come from more recent genetic studies. In the proximal tubule and thick ascending limb, magnesium reabsorption is believed to occur primarily through the paracellular shunt pathway, which ultimately depends on the electrochemical gradient setup by active sodium reabsorption. In the distal convoluted tubule, magnesium transport is transcellular, although magnesium reabsorption also appears to be related to active sodium reabsorption in this segment. In addition, evidence suggests that magnesium transport is highly regulated, although a specific hormonal regulator of extracellular magnesium has yet to be identified.

Corticomedullary difference in the effects of dietary Ca²⁺ on tight junction properties in thick ascending limbs of Henle's loop

The thick ascending limb of Henle's loop (TAL) drives an important part of the reabsorption of divalent cations. This reabsorption occurs via the paracellular pathway formed by the tight junction (TJ), which in the TAL shows cation selectivity. Claudins, a family of TJ proteins, determine the permeability and selectivity of this pathway. Mice were fed with normal or high-Ca(2+) diet, and effects on the reabsorptive properties of cortical and medullary TAL segments were analysed by tubule microdissection and microperfusion. Claudin expression was investigated by immunostaining and quantitative PCR. We show that the TAL adapted to high Ca(2+) load in a sub-segment-specific manner. In medullary TAL, transcellular NaCl transport was attenuated. The transepithelial voltage decreased from 10.9 ± 0.6 mV at control diet to 8.3 ± 0.5 mV at high Ca(2+) load, thereby reducing the driving force for Ca(2+) and Mg(2+) uptake. Cortical TAL showed a reduction in paracellular Ca(2+) and Mg(2+) permeabilities from 8.2 ± 0.7 to 6.2 ± 0.5 ∙ 10(-4) cm/s and from 4.8 ± 0.5 to 3.0 ± 0.2 · 10(-4) cm/s at control and high-Ca(2+) diet, respectively. Expression, localisation and regulation of claudins 10, 14, 16 and 19 differed along the corticomedullary axis: Towards the cortex, the main site of divalent cation reabsorption in TAL, high-Ca(2+) intake led to a strong upregulation of claudin-14 within TAL TJs while claudin-16 and -19 were unaltered. Towards the inner medulla, only claudin-10 was present in TAL TJ strands. In summary, high-Ca(2+) diet induced a reduction of divalent cation reabsorption via a diminution of NaCl transport and driving force in mTAL and via decreased paracellular permeabilities in cTAL. We reveal an important regulatory pattern along the corticomedullary axis and improve the understanding how the kidney disposes of detrimental excess Ca(2+).

Paracellular calcium transport across renal and intestinal epithelia

Calcium (Ca(2+)) is a key constituent in a myriad of physiological processes from intracellular signalling to the mineralization of bone. As a consequence, Ca(2+) is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for Ca(2+) from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial Ca(2+) flux. More specifically, passive paracellular Ca(2+) absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle's loop. Importantly, recent studies demonstrated that Ca(2+) transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular Ca(2+) flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular Ca(2+) movement and the identity of claudins that permit the passage of Ca(2+) through the tight junction of these epithelia.

Increased expression of renal TRPM6 compensates for Mg(2+) wasting during furosemide treatment

Background

Furosemide is a loop diuretic, which blocks the Na⁺, K⁺, 2Cl⁻ cotransporter (NKCC2) in the thick ascending limb of Henle (TAL). By diminishing sodium (Na⁺) reabsorption, loop diuretics reduce the lumen-positive transepithelial voltage and consequently diminish paracellular transport of magnesium (Mg²⁺) and calcium (Ca²⁺) in TAL. Indeed, furosemide promotes urinary Mg²⁺ excretion; however, it is unclear whether this leads, especially during prolonged treatment, to hypomagnesaemia. The aim of the present study was, therefore, to determine the effect of chronic furosemide application on renal Mg²⁺ handling in mice.

Methods

Two groups of 10 mice received an osmotic minipump subcutaneously for 7 days with vehicle or 30 mg/kg/day furosemide. Serum and urine electrolyte concentrations were determined. Next, renal mRNA levels of the epithelial Mg²⁺ channel (TRPM6), the Na⁺, Cl⁻ cotransporter (NCC), the epithelial Ca²⁺ channel (TRPV5), the cytosolic Ca²⁺-binding protein calbindin-D_28K, as well parvalbumin (PV), claudin-7 (CLDN7) and claudin-8 (CLDN8), the epithelial Na⁺ channel (ENaC) and the Na⁺–H⁺ exchanger 3 (NHE3) were determined by real-time quantitative polymerase chain reaction. Renal protein levels of NCC, TRPV5, calbindin-D_28K and ENaC were also measured using semi-quantitative immunohistochemistry and immunoblotting.

Results

The mice chronically treated with 30 mg/kg/day furosemide displayed a significant polyuria (2.1 ± 0.3 and 1.3 ± 0.2 mL/24 h, furosemide versus control respectively, P < 0.05). Furosemide treatment resulted in increased serum concentrations of Na⁺ [158 ± 3 (treated) and 147 ± 1 mmol/L (control), P < 0.01], whereas serum K⁺, Ca²⁺ and Mg²⁺ values were not significantly altered in mice treated with furosemide. Urinary excretion of Na⁺, K⁺, Ca²⁺ and Mg²⁺ was not affected by chronic furosemide treatment. The present study shows specific renal upregulation of TRPM6, NCC, TRPV5 and calbindin-D_28K.

Conclusions

During chronic furosemide treatment, enhanced active reabsorption of Mg²⁺ via the epithelial channel TRPM6 in DCT compensates for the reduced reabsorption of Mg²⁺ in TAL.

Claudin-16 and claudin-19 function in the thick ascending limb

PURPOSE OF REVIEW

Claudin-16 and claudin-19 play a major role in the regulation of magnesium reabsorption in the thick ascending limb (TAL). This review describes recent findings of the physiological function of claudin-16 and claudin-19 underlying normal transport function for magnesium reabsorption in the TAL.

RECENT FINDINGS

Mutations in the genes encoding the tight junction proteins claudin-16 and claudin-19 cause the inherited human renal disorder familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The cation selectivity of the tight junction is vital for generating the lumen positive transepithelial potential in the TAL, which drives paracellular absorption of magnesium. Claudin-16 and claudin-19 require each other for assembly into tight junctions in the TAL. Heteromeric claudin-16 and claudin-19 interaction forms a cation selective tight junction paracellular channel. Loss of either claudin-16 or claudin-19 in the mouse kidney abolishes the cation selectivity for the TAL paracellular pathway, leading to excessive renal wasting of magnesium.

SUMMARY

Epithelial paracellular channels are increasingly understood to be formed from claudin oligomeric complexes. In the mouse TAL, claudin-16 and claudin-19 cooperate to form cation-selective paracellular channels required for normal levels of magnesium reabsorption. Different subsets of the claudin family of tight junction proteins are found distributed throughout the nephron, and understanding their roles in paracellular ion transport will be fundamental to understanding renal ion homeostasis.

Claudin-16 affects transcellular Cl- secretion in MDCK cells

Claudin-16 (paracellin-1) is a tight junction protein localized mainly in the thick ascending limb of Henle's loop and also in the distal nephron. Its defect causes familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. This had been taken as an indication that claudin-16 conveys paracellular Mg(2+) and Ca(2+) transport; however, evidence is still conflicting. We studied paracellular ion permeabilities as well as effects of claudin-16 on the driving forces for passive ion movement. MDCK-C7 cells were stably transfected with wild-type (wt) and mutant (R146T, T233R) claudin-16. Results indicated that paracellular permeability to Mg(2+) but not to Ca(2+) is increased in cells transfected with wt compared to mutant claudin-16 and control cells. Increased basolateral Mg(2+) concentration activated a transcellular Cl(-) current which was greatly enhanced in cells transfected with wt and T233R claudin-16, as compared to R146T claudin-16-transfected or control cells. This current was triggered by the basolateral calcium-sensing receptor causing Ca(2+) release from internal stores, thus activating apical Ca(2+)-sensitive Cl(-) channels and basolateral Ca(2+)-sensitive K(+) channels. Immunohistochemical data suggest that the Cl(-) channel involved is bestrophin. We conclude that claudin-16 itself possesses only moderate paracellular Mg(2+) permeability but governs transcellular Cl(-) currents by interaction with apical Ca(2+)-activated Cl(-) channels, presumably bestrophin. As the transepithelial voltage generated by such a current alters the driving force for all ions, this may be the major mechanism to regulate Mg(2+) and Ca(2+) absorption in the kidney.

Function and regulation of claudins in the thick ascending limb of Henle

The thick ascending limb (TAL) of Henle mediates transcellular reabsorption of NaCl while generating a lumen-positive voltage that drives passive paracellular reabsorption of divalent cations. Disturbance of paracellular reabsorption leads to Ca(2+) and Mg(2+) wasting in patients with the rare inherited disorder of familial hypercalciuric hypomagnesemia with nephrocalcinosis (FHHNC). Recent work has shown that the claudin family of tight junction proteins form paracellular pores and determine the ion selectivity of paracellular permeability. Importantly, FHHNC has been found to be caused by mutations in two of these genes, claudin-16 and claudin-19, and mice with knockdown of claudin-16 reproduce many of the features of FHHNC. Here, we review the physiology of TAL ion transport, present the current view of the role and mechanism of claudins in determining paracellular permeability, and discuss the possible pathogenic mechanisms responsible for FHHNC.

Gender differences in kidney function

Sex hormones influence the development of female (F) and male (M) specific traits and primarily affect the structure and function of gender-specific organs. Recent studies also indicated their important roles in regulating structure and/or function of nearly every tissue and organ in the mammalian body, including the kidneys, causing gender differences in a variety of characteristics. Clinical observations in humans and studies in experimental animals in vivo and in models in vitro have shown that renal structure and functions under various physiological, pharmacological, and toxicological conditions are different in M and F, and that these differences may be related to the sex-hormone-regulated expression and action of transporters in the apical and basolateral membrane of nephron epithelial cells. In this review we have collected published data on gender differences in renal functions, transporters and other related parameters, and present our own microarray data on messenger RNA expression for various transporters in the kidney cortex of M and F rats. With these data we would like to emphasize the importance of sex hormones in regulation of a variety of renal transport functions and to initiate further studies of gender-related differences in kidney structure and functions, which would enable us to better understand occurrence and development of various renal diseases, pharmacotherapy, and drug-induced nephrotoxicity in humans and animals.

Cyclic guanosine monophosphate is the mediator of platelet-activating factor inhibition on transport by the mouse kidney thick ascending limb

Since we have previously shown a direct inhibitory effect of platelet-activating factor (PAF) on Cl reabsorption in the medullary thick ascending limb of Henle's loop (TAL), the aim of this study was to extend this effect to the whole TAL and to further investigate the signaling pathway involved. In microperfused cortical TALs, PAF significantly decreased Cl reabsorption by 50.3 +/- 6.5%. On the one hand, this effect was not modified in the presence of staurosporine and was not mimicked by phorbol ester; chelating cytosolic Ca by BAPTA/AM failed to suppress the inhibitory effect of PAF on Cl reabsorption; moreover, no significant increase in intracellular Ca concentration could be observed in the presence of PAF on isolated tubules. On the other hand, 8-bromo cyclic GMP mimicked the PAF effect on Cl reabsorption and prevented a further effect of this agent; the PAF effect was significantly reduced by H-8, a cyclic GMP-dependent protein kinase inhibitor; in medullary TALs, PAF significantly increased by twofold cyclic GMP content, an effect inhibited by the PAF antagonist BN 50730, whereas PAF did not significantly modify cAMP content in basal or stimulated conditions. Finally, inhibition of nitric oxide production by NAME or NMMA failed to prevent the effect of PAF on Cl reabsorption. It is concluded that the PAF-induced inhibition of Cl reabsorption in the TAL was mediated by cyclic GMP, likely independent of a nitric oxide synthesis.

Ca2+, Mg2+ and K+ transport in the cortical and medullary thick ascending limb of the rat nephron: influence of transepithelial voltage

Isolated segments of rat cortical (cTAL) and medullary (mTAL) thick ascending limbs were microperfused and the transepithelial net fluxes (JX) were determined by measuring the composition of the collected fluid with an electron microprobe. When perfused with symmetrical solutions both segments showed similar JNa and JCl and lumen-positive transepithelial voltage (Vte = 7-8 mV). JMg, JCa and JK were not significantly different from zero. When perfused with asymmetrical solutions (lumen 50 mM, bath 150 mM NaCl), the mean Vte were 23 mV and 17 mV in the cTAL and mTAL respectively; this rise was accompanied by significant increases in JMg and JCa in the cTAL, but not in the mTAL, and a marked increase in JK in both segments. It is concluded that, in the rat, divalent cations can be reabsorbed in the cTAL, and K+ can be reabsorbed in the cTAL and mTAL. The transport is voltage-dependent. The mTAL can reabsorb neither Mg2+ nor Ca2+, whatever Vte.

Hormonal stimulation of Ca2+ and Mg2+ transport in the cortical thick ascending limb of Henle's loop of the mouse: evidence for a change in the paracellular pathway permeability

Recent studies from our laboratory have shown that in the cortical thick ascending limb of Henle's loop of the mouse (cTAL) Ca2+ and Mg2+ are reabsorbed passively, via the paracellular shunt pathway. In the present study, cellular mechanisms responsible for the hormone-stimulated Ca2+ and Mg2+ transport were investigated. Transepithelial voltages (PDte) and transepithelial ion net fluxes (JNa, JCl, JK, JCa, JMg) were measured in isolated perfused mouse cTAL segments. Whether parathyroid hormone (PTH) is able to stimulate Ca2+ and Mg2+ reabsorption when active NaCl reabsorption and thus PDte, is abolished by luminal furosemide was first tested. With symmetrical lumen and bath Ringer's solutions, no Ca2+ and Mg2+ net transport was detectable, either in the absence or in the presence of PTH. In the presence of luminal furosemide and a chemically imposed lumen-to-bath directed NaCl gradient, which generates a lumen-negative PDte, PTH slightly but significantly increased Ca2+ and Mg2+ net secretion. In the presence of luminal furosemide and a chemically imposed bath-to-lumen-directed NaCl gradient, which generates a lumen-positive PDte, PTH slightly but significantly increased Ca2+ and Mg2+ net reabsorption. In view of the observed small effect of PTH on passive Ca2+ and Mg2+ movement, a possible interference of furosemide with the hormonal response was considered. To investigate this possibility, Ca2+ and Mg2+ transport was first stimulated with PTH in tubules under control conditions. Then active NaCl reabsorption was abolished by furosemide and the effect of PTH on JCa and JMg measured. In the absence of PDte and under symmetrical conditions, no Ca2+ and Mg2+ transport was detectable, either in the presence or absence of PTH. In the presence of a bath-to-lumen-directed NaCl gradient, Ca2+ and Mg2+ reabsorption was significantly higher in the presence than in the absence of PTH. Finally, when active NaCl transport was not inhibited by furosemide, but reduced by a bath-to-lumen-directed NaCl gradient, PTH strongly increased JCa and JMg, whereas only a small increase in PDte was noted. In conclusion, these data suggest that PTH exerts a dual action on Ca2+ and Mg2+ transport in the mouse cTAL by increasing the transepithelial driving force for Ca2+ and Mg2+ reabsorption through hormone-mediated PDte alterations and by modifying the passive permeability for Ca2+ and Mg2+ of the epithelium, very probably at the level of the paracellular shunt pathway.